- Preservation from microorganisms: Contamination due to microorganisms can result in problems such as color and odor change, gas production, hydrolysis, pH change and eventually breaking down of emulsion. Therefore, it is necessary that emulsions are adequately preserved.An ideal preservative should be nonirritant, nonsensitizing and nontoxic in the concentration used. It should be physically as well as chemically compatible with the other ingredients of the emulsions and with the container of the product. It should not impart any taste, color or odor to the product. The preservative should be stable and effective over a wide range of pH and temperature. It should have a wide spectrum of activity against a range of bacteria, yeasts and molds. The selective preservative should have high water solubility and a low oil–water partition coefficient. It should have bactericidal rather than bacteriostatic activity.Examples of antimicrobial preservatives used to preserve emulsified systems include p-hydroxybenzoate esters such as methyl, propyl and butyl parabens, organic acids such as ascorbic acid and benzoic acid, organic mercurials such as phenylmercuric acetate and phenylmercuric nitrate, quaternary ammonium compounds such as cetrimide, cresol derivatives such as chlorocresol and miscellaneous agents such as sodium benzoate, chloroform and phenoxyethanol.
- Preservation from oxidation: Oxidative changes such as rancidity and spoilage due to atmospheric oxygen and the effects of enzymes produced by microorganisms are seen in many emulsions containing vegetable and mineral oils and animal fats. Antioxidants can be used to prevent the changes occurring due to atmospheric oxygen.The ideal antioxidant should be soluble in the vehicle, nontoxic, nonirritant and effective at low concentration under the expected conditions of storage and use. Antioxidants for use in oral preparation should also be odorless and tasteless.Some of the commonly used antioxidants for emulsified systems are ethyl gallate, propyl gallate, dodecyl gallate, butylated hydroxyanisole (BHT) and butylated hydroxytoluene (BHT)
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